1. Field of Invention
The present invention relates to a surface light source device of side light type and a liquid crystal display, in particular, to a surface light source device of side light type which is applied to an auxiliary illumination arrangement in a liquid crystal display and to a liquid crystal display equipped with the light source device.
2. Related Art
Liquid crystal displays of a known type employ a transmission-type liquid crystal and ones of another known type employ a reflection-type liquid crystal. In general, the latter permits consumption of electric power to be much saved as compared with the former, because ambient light can be utilized for image formation. And it is known that an auxiliary illumination device is mounted on the liquid crystal display to enable the display to perform displaying under the condition of short ambient light such as at night.
So-called surface light source devices of the side light type are suitable for such an application. This is because of their structure such that a primary light source and a guide plate are arranged side by side to add only a very small thickness to the liquid crystal display. It is known that a reflection-type liquid crystal may function supplementally as a transmission-type liquid crystal when a surface light source device of side light type applied to auxiliary illumination is switched on.
FIG. 8 and FIG. 9 illustrate an example of conventional surface light source device of side light type which is structured according to the above manner. Referring to FIGS. 8 and 9, a liquid crystal display 1 comprises a surface light source device of side light type arranged behind a liquid crystal display panel 2. That is, the surface light source device 3 provides an auxiliary illumination means. The liquid crystal display 1 is viewed from above FIGS. 8 and 9.
The liquid crystal display panel 2 comprises a transmission-reflection plate 5, a polarization plate 6, a glass substrate 7, a liquid crystal layer 8, another glass substrate 9 and another polarization plate 10 which are disposed laminatedly in order. In these members, the transmission-reflection plate 5 is located at the closest position to the surface light source device 3.
Transparent electrodes are formed on surfaces of the glass substrates 7 and 9, respectively, providing transparent electrode patterns between which a liquid crystal material is interposed and sealed to form the liquid crystal layer 8.
The liquid crystal display panel 2 provides liquid crystal cells with matrix-like arrangement and each of the cells rotates a polarization plane of light transmitting therethrough depending on voltage applied to the transparent electrodes.
In general, the polarization plates 6 and 10 are orientated so that the transmission polarization plane of the polarization plate 6 is either perpendicular or parallel to that of the polarization plate 10.
Each of the polarization plates 6 and 10 permits a component corresponding to a certain direction of polarization plane (transmission polarization plane) to transmit exclusively.
The transmission-reflection plate 5 is a member provided with both transmissivity and reflectivity, being arranged so that scattering power and high reflectance is demonstrated with respect to light which is transmitted through the liquid crystal cells while being demonstrated high transmissivity with respect to light which is supplied from the surface light source device 3.
On impinging of ambient light L1 to the liquid crystal display panel from above in FIG. 9, some component of the ambient light transmits through the polarization plate 10, the glass substrate 9, the liquid crystal layer 8, the glass substrate 7 and the polarization plate 6 to reach the transmission-reflection plate 5.
Quantity of light reaching the transmission-reflection plate 5 depends on factors including directions of transmission polarization planes of the polarization plates 6, 10 and state of the liquid crystal layer 8 (depending on voltage applied to the transparent electrodes).
The transmission-reflection plate 5 scatters and reflects ambient light L1. However, it involves partial transmission therethrough, because the transmission-reflection plate 5 is provided with transmissivity as well.
Some component of the scattered and reflected light transmits through the polarization plate 6, the glass substrate 7, the liquid crystal layer 8, the glass substrate 9 and the polarization plate 10 in order, being emitted toward the outside to contribute to displaying.
Quantity of light emitting toward the outside varies depending on factors including directions of transmission polarization planes of the polarization plates 10, 6 and state of the liquid crystal layer 8 (depending on voltage applied to the transparent electrodes).
According to this principle, bright-dark distribution is formed to provide image to be viewed.
The surface light source device 3 is switched on usually when ambient light is short. In the illustrated arrangement, the surface light source device 3 functions as a back-lighting means. The surface light source device 3 comprises a guide plate 12 and a primary light source disposed beside the guide plate.
The primary light source 13 is composed of, for instance, a fluorescent lamp (cold cathode lamp) 14 and a reflector 15 backing the lamp. When the fluorescent lamp 14 is turned on, primary light is introduced into the guide plate 12 through an opening of the reflector 15 and an end face 12A of the guide plate. The guide plate 12 is, for example, made of a transparent resin such as acrylic resin. Alternatively, a scattering guide plate having scattering power inside is employed occasionally.
In the illustrated case, the guide plate 12 has a roughened back face (the lower face in FIG. 9). Illumination light L2 introduced into the guide plate 12 is diffused and reflected at the back face and is emitted from an illumination output face (the upper face in FIG. 9) on the way of propagation. A reflection member (not shown) is disposed along the back face of the guide plate 12 occasionally.
As required, an additional member such as a prism sheet or a light diffusion sheet is disposed along the illumination output face of the guide plate 12 in order to modify directivity of emission.
Some of the outputted illumination light from the surface light source device 3 transmits through the transmission-reflection plate 5. Then some component of it is emitted toward the outside after transmitting along paths (polarization plate 6 glass substrate 7 liquid crystal layer 8 glass substrate 9 polarization plate 10) similar to those of ambient light L1, thereby contributing to displaying.
Quantity of light emitting toward the outside varies, in the same manner as that of ambient light L1, depending on factors including directions of transmission polarization planes of the polarization plates 10, 6 and state of the liquid crystal layer 8 (depending on voltage applied to the transparent electrodes). According to this principle, bright-dark distribution is formed to provide image to be viewed.
A problem with the above-described conventional liquid crystal display 1 is that ambient light is utilized at a low efficiency. This is because the transmission-reflection plate 5 disposed at the back side of the polarization plate 6 for auxiliary illumination has transmissivity which allow some of ambient light L1 to be leaked out. Needless to say, this leakage leads to reduction in quantity of light contributing to displaying, thereby lowering contrast of display. In particular, color image displaying LCDs will suffer from an inferior display quality brought by short brightness and low contrast.